Lead or lead alloy anodes have been widely used in processes for electrowinning metals from sulphate solutions. They nevertheless have important limitations, such as a high oxygen overvoltage and loss of the anode material leading to contamination of the electrolyte as well as the metal product obtained on the cathode.
Anodes of lead-silver alloy provide a certain decrease of the oxygen overvoltage and improvement of the current efficiency, but they still have the said limitations as a whole.
It has been proposed to use dimensionally stable titanium anodes with a platinum metal oxide coating for anodic evolution of oxygen, but such anodes are generally subject to more or less rapid passivation and oxidation of the titanium base.
It has also been proposed to provide the titanium base with a protective undercoating comprising a platinum group metal beneath the outer coating, but they generally do not provide sufficient protection to justify the high cost of using precious metals.
Metal electrowinning cells generally require a large anode surface in order to ensure an even electrodeposition on the cathode, so that the cost of using a titanium base must also be taken into account.
Dimensionally stable anodes with mixed oxide coatings comprising platinum group metals and valve metals are described in U.S. Pat. 3 632 498. An example of this patent relates to the preparation of a fine Ti-Pd mixed oxide power which is then applied by rolling or hammering into a rod of soft-quality titanium. However, the amount of precious metal incorporated in the mixed oxide powder and applied to the electrode in this manner could be prohibitive for various industrial applications. Thus, when the electrode surface is to be substantially covered with the mixed oxide powder, and more particularly when the electrode is intended for operation at a relatively low current density such as is used in metal electrowinning, the cost of precious metal thus applied in the form of a mixed oxide may be especially prohibitive.